Constructed Wetland Treatment of Tile Drainage - New Zealand

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Constructed Wetland Treatment of Tile Drainage - New Zealand
New Zealand
   Constructed
Wetland Treatment
 of Tile Drainage
    New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   1
Constructed Wetland Treatment of Tile Drainage - New Zealand
New Zealand
Constructed Wetland
  Treatment of Tile
      Drainage                 Authors:     Chris C. Tanner
                                            James P. S. Sukias
                                            Charlotte R. Yates
                     Acknowledgments:       This guide was developed with funding from the Foundation for Research, Science and Technology (FRST) under Contract No. CO1X0304. The guidelines
                                            are based on information from a range of field and experimental studies funded through this and preceding FRST contracts; by the New Zealand dairy
                                            industry (Dairy Insight, Dexcel, Fonterra, Dairy Research Institute); and by the Department of Conservation. We thank NIWA and AgResearch staff who have
                                            assisted with monitoring, Paul Champion (NIWA) for advice on weed control, and the farmers who have allowed trials to be carried out on their farms, and
                                            assisted with wetland construction and management. Photos are mainly those of the authors. Additional photos provided by Rohan Wells, Paul Champion,
                                            Mary deWinton (NIWA).

                               Citation:    Tanner, C.C.; Sukias, J.P.S.; Yates, C.R. (2010). New Zealand guidelines: Constructed Wetland Treatment of Tile Drainage. NIWA Information Series No. 75
                                            National Institute of Water & Atmospheric Research Ltd.

             New Zealand Guidelines for     NIWA Information Series No. 75
                   Constructed Wetland      © NIWA 2010
              Treatment of Tile Drainage    ISSN 1174-264X
                      www.niwa.co.nz        http://www.niwa.co.nz/our-science/freshwater/tools/tile-drain-wetland-guidelines

           Contact for more information:    National Institute of Water & Atmospheric Research Ltd (NIWA)
                                            Chris C. Tanner (c.tanner@niwa.co.nz)
                                            PO Box 11115, Hillcrest, Hamilton 3251, New Zealand

                                            © 2010 – National Institute of Water & Atmospheric Research Limited. All rights reserved.
                                            Unless otherwise stated, the contents of this publication (including without limitation all text, diagrams, photographs, compilation of data or other materials) are subject to
                                            copyright. Copyright is owned by the National Institute of Water & Atmospheric Research Limited (“NIWA”). This publication may not be reproduced, adapted, or distributed,
                                            in whole or in part, or used for any commercial purpose without NIWA’s express written permission.
                                            While NIWA has used all reasonable endeavours to ensure that the information contained in this publication is accurate, no expressed or implied warranty is given by NIWA
                                            as to the completeness or accuracy of the information provided or that the guidelines contained herein will be appropriate for all situations. As the publication and
                                            information contained herein is of a general nature NIWA recommends that you seek specific advice, either from NIWA or other relevant industry professionals, regarding
                                            your intended use(s). Accordingly, you agree that if you use, or rely upon, any information within this publication NIWA will not be liable for any claim, loss, cost or damage
                                            incurred or suffered whatsoever. NIWA also recommends that prior to construction of a wetland you contact your local consenting authority to determine compliance with the
                                            Resource Management Act and/or all applicable local or regional regulations and plans.

                          Graphic design:   Aarti Wadhwa

                                                                                                                                     New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage                2
Constructed Wetland Treatment of Tile Drainage - New Zealand
Contents
           Purpose ........................................................................................................................................................................................................................... 1

           Background ................................................................................................................................................................................................................... 2

           Getting started .............................................................................................................................................................................................................     5
           Stage 1 Choose location ……………………………………………………………………………………………………                                                                                                                                                                    6
                          Scenario 1: End of tile drains, alongside surface drains and stream ............................................................................                                                                   7
                          Scenario 2: End of surface drains supplied by tile drains .................................................................................................                                                        8
           Stage 2 Design ………………………………………………………………………………………………………………                                                                                                                                                                         9
                       A. Size
                          Constructed wetland size and treatment performance ...................................................................................................                                                           10
                          Calculating the wetland area required ...................................................................................................................................                                        11
                       B. Layout .................................................................................................................................................................................................         12
                       C. Embankments and sealing ..........................................................................................................................................................                               13
                       D. Shape
                          Constructed wetland shape ........................................................................................................................................................                               14
                          Alternative constructed wetland shapes ...............................................................................................................................                                           15
                          Constructed wetland layout example .....................................................................................................................................                                         16
                       E. Inlet structures .................................................................................................................................................................................               17
                          Wetland width up to 8 m .............................................................................................................................................................                            18
                          Wetland width 8–30 m ..................................................................................................................................................................                          19
                          Wetland width over 30 m ...........................................................................................................................................................                              20
                       F. Sediment traps
                          Where wetland may receive sediment-rich inflows ...........................................................................................................                                                      21

                                                                                                                             New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage                                             3
Constructed Wetland Treatment of Tile Drainage - New Zealand
Contents
                       G. Outlet and water-level control structures .............................................................................................................................                              22
                       H. Phosphorus retaining additives ..............................................................................................................................................                        24
           Stage 3 Construction ………………………………………………………………………………………………………                                                                                                                                                        25
                       A. Timeline/planning .........................................................................................................................................................................          26
                       B. Construction day and beyond ...................................................................................................................................................                      27
           Stage 4 Establish plants ……………………………………………………………………………………………………                                                                                                                                                     28
                       A. Role of plants ...................................................................................................................................................................................   29
                       B. Planting zones .................................................................................................................................................................................     30
                       C. Plant selection
                          Key species for the flooded zone ..............................................................................................................................................                      31
                              Typha orientalis (raupo, bulrush) .......................................................................................................................................                        32
                              Baumea articulata (mokuautoto, jointed twig-rush) ..................................................................................................                                             33
                              Eleocharis sphacelata (kuta, tall spike-rush) ...................................................................................................................                                34
                              Schoenoplectus tabernaemontani (kapungawha, soft-stem bulrush) ..................................................................                                                                35
                          Key species for shallow-water margins and embankments ...........................................................................................                                                    36
                       D. Planting and establishment .......................................................................................................................................................                   38
           Stage 5 Maintenance ………………………………………………………………………………………………………                                                                                                                                                         41
                       A. Requirements during wetland establishment ....................................................................................................................                                       42
                       B. Requirements after wetland establishment .........................................................................................................................                                   43

           Appendices …………………………………………………………………………………………………………………                                                                                                                                                              44
                     Appendix 1: Specialist wetland plant suppliers in New Zealand .....................................................................................                                                       44
                     Appendix 2: Resources for wetland plant identification and management .................................................................                                                                   45
                     Appendix 3: Herbicides for weed control during wetland establishment and maintenance .................................                                                                                    46

           References ………………………………………………………………………………………………………………….. 47

           Glossary …………………………………………………………………………………………………………………….. 48

                                                                                                                        New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage                                   4
Constructed Wetland Treatment of Tile Drainage - New Zealand
Purpose
          This guide provides information on how to design and construct wetlands to
          intercept and treat tile drainage flows from rain-fed grazed pasture in
          New Zealand. The focus is on removal of nitrate-nitrogen loads. Information to
          estimate potential nitrogen removal is based on the results of field studies and
          modelling.

          What is the purpose of    The New Zealand Guidelines for Constructed Wetland Treatment of Tile
          these guidelines?         Drainage are intended to guide farmers, farm advisors, rural contractors,
                                    and regional council staff to appropriately locate, size, design, and construct
                                    effective treatment wetlands. They also provide guidance on wetland planting,
                                    weed control, and maintenance in order to achieve effective treatment of tile
                                    drainage.

          Why build a constructed   Tile drainage can act as a significant route for nutrient losses, particularly
          wetland to treat tile     of nitrogen, from intensively grazed pastures to waterways. Current annual
          drainage flow?            nitrogen losses in tile drainage flows from intensively grazed dairy pastures in
                                    New Zealand commonly range from 20–60 kg N per hectare, predominantly as
                                    nitrate. This quantity of nitrogen can pose a significant threat to water quality
                                    in sensitive receiving streams, rivers, lakes, and estuaries. Alternative design
                                    and management of tile drainage systems is required in these situations to
                                    integrate environmental goals with traditional agricultural water and soil
                                    management goals.

                                    Constructed wetlands offer a potentially cost-effective, practical option for
                                    intercepting highly variable tile drainage flows to reduce nutrient losses
                                    from pastures, before they reach receiving water bodies. They should ideally
                                    be employed in combination with good fertiliser, grazing, and effluent
                                    management practices.

                                                           New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   1
Constructed Wetland Treatment of Tile Drainage - New Zealand
Background
             Constructed wetlands have been recognised in many countries as an effective
             technology for treatment of tile drainage waters. As a best management practice,
             they capitalise on natural processes to reduce pollution from incoming water,
             while enhancing local biodiversity values.

             What information are     The recommendations in these guidelines are based on the results of
             these guidelines based   farm-scale trials and modelling carried out by NIWA based on five years
                                      of monitoring in the Waikato, and three and four years of monitoring in
             on?
                                      Northland and Southland, respectively (Sukias, J.P.S. et al. (2006), Tanner, C.C. et
                                      al. (2005a), Tanner, C.C. et al. (2005b); Tanner & Sukias, (2010)). In combination
                                      with international experience, these results have allowed the quantification
                                      of nitrogen removal in relation to wetland size and design, providing realistic
                                      guidance on performance expectations for land managers and regulators. The
                                      constructed wetland performance predictions presented apply primarily to
                                      tile-drained areas of key dairying regions in Northland, Waikato, Bay of Plenty,
                                      Taranaki, and Manawatu, with rainfall in the range of 800–1400 mm per year.
                                      Estimated differences in performance for Southland and Otago, where there
                                      are also significant areas of tile-drained dairy pastures, are also given.

             What are constructed     Natural wetlands have been called the “kidneys of the landscape” because of
                                      their ability to store, assimilate, and transform contaminants lost from the land,
             wetlands?
                                      before they reach waterways. Constructed wetlands attempt to mimic these
                                      natural systems to treat through-flowing waters. Generally shallow surface-
                                      flow wetlands are the most appropriate type of constructed wetland system
                                      for treatment of farm tile drainage. They are similar to natural marshes and
                                      swamps, in which water flows through shallow flooded beds of emergent
                                      aquatic plants such as raupo (Typha orientalis).

                                                              New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   2
Constructed Wetland Treatment of Tile Drainage - New Zealand
Background

                                  In some cases, naturally occurring wetlands on farms can be used to intercept
                                  and treat farm drainage. Constructed wetlands have the advantage that they
                                  can be created where they are required, avoiding the risk of contamination
                                  and degradation of high quality natural wetland habitats.

             How do constructed   Flows are dispersed and slowed down when they enter wetlands, promoting
             wetlands work?       settling and deposition of suspended particles. Wetlands are also highly
                                  productive environments for plants and beneficial microbes which break
                                  down, assimilate and cycle nutrients, organic matter, and associated pollutants,
                                  transforming them into less harmful forms. In particular, denitrifying bacteria
                                  provide an important means for sustainably removing nitrate-nitrogen by
                                  converting it into nitrogen gas that is released back into the atmosphere.

             What environmental   Rainfall patterns, soil water status, groundwater levels, soil properties,
             factors affect       drainage system design, and land-management practices affect the flows and
             performance?         contaminant loads generated in farm tile drainage. Consequently, tile drainage
                                  flow rates can vary from a trickle to a deluge, and contaminant concentrations
                                  fluctuate widely from hour to hour and day to day. Drainage flows can also
                                  vary significantly between sites, seasons and years, and across different
                                  regions of the country. Depending on the relative size and design of the
                                  wetland, the resulting residence time (average amount of time drainage water
                                  spends in the wetland) may also vary widely. The biological removal processes
                                  operating in wetlands are most efficient when residence times are long and
                                  water temperatures are high. Performance tends to be poorer during cold,
                                  wet, winter periods. Such factors significantly affect the capacity of wetlands
                                  to remove nitrate-nitrogen and other contaminants. Because of this inherent
                                  variability, we have focused on providing estimates of long-term annual
                                  average percentage removal performance, and indicated the normal range of
                                  performance that can be expected in typical dairying regions of New Zealand.
                                                         New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   3
Constructed Wetland Treatment of Tile Drainage - New Zealand
Background

             Situations these guidelines 1] Tile-drains conveying irrigated dairy effluent
             do not apply to:               Tile drains can act as a rapid conduit for irrigated farm dairy effluent.
                                               Depending on soil characteristics and preceding soil water status,
                                               drainage from such areas may contain high concentrations of organic
                                               matter, nitrogen in the form of ammonium, and faecal microbes.
                                               Although constructed wetland treatment will likely improve such
                                               discharges (Tanner, C.C.; Kloosterman, V.C. (1997)), the sizing, design, and
                                               performance estimates provided in these guidelines do not apply to such
                                               situations.

                                             2] Large-scale constructed wetlands
                                                The information presented in these guidelines is relevant for constructed
                                                wetlands up to 1 hectare (10,000 m2) in size in low gradient sites and
                                                stable soil conditions. For projects larger than this, and any situation
                                                where there is potential for land instability or flooding, it is recommended
                                                that you seek professional engineering advice.

             Compliance with RMA             Please check with your local regional council regarding relevant rules
                                             and regulations. Construction of dams, culverts, or earthworks in or near
                                             waterways, in particular, may be subject to regional rules and/or require
                                             resource consent under the Resource Management Act 1991.

             Structural integrity            Use of these guidelines does not in any way negate the need to seek
                                             professional engineering advice to ensure sound and appropriate design
                                             and construction, particularly for large-scale applications and where there is
                                             potential for land instability or flooding.

                                                                    New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   4
Constructed Wetland Treatment of Tile Drainage - New Zealand
Getting started    It is critical to plan the implementation of the project, including timing of
                   key stages, BEFORE starting your constructed wetland.

              There are five main stages to constructing a wetland:

                           Stage 1   Choose location

                           Stage 2   Design

                           Stage 3   Construction

                           Stage 4   Establish plants

                           Stage 5   Maintenance

                                                     New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   5
Constructed Wetland Treatment of Tile Drainage - New Zealand
Stage 1                          Choose location
This section provides guidance
for siting the constructed
wetland.

                                               New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   6
Stage 1Choose location
                                                          Choosing a site to build your constructed wetland will depend on the configuration and existing functions of
                                                          the available land and where the drainage system can best be intercepted. Likely areas include:
      What if the area available
                                                          t at the downstream end of tile drains, alongside surface drains and streams,
      is limited?                                         t at the end of surface drains that are supplied by tile drains.
                                                                                                                                                                  Tile drains rerouted to
                                                          Providing access to the wetland to enable future maintenance
      It may be possible to                               is important.
                                                                                                                                                                  pass through CW before
                                                                                                                                                                  entering into stream.
      divide the wetland into
      separate sections with
      one flowing into another.                           Scenario 1: End of tile drains,
      For example, if a farm                              alongside surface drains and streams
      track divides the area, a
                                                          An appropriate place to intercept tile drainage and construct
      subsurface pipe could link                          wetlands is alongside receiving drains or streams. This provides
      the sections on either side.                        valuable wetland habitat closely linked to waterways, and reduces
                                                          potential interference with farming activities.

A newly installed culvert linking two wetland sections.   Intercepting a buried tile drain.

                                                                                                                 New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   7
Stage 1Choose location
          Surface drain                Scenario 2: End of surface drains supplied by tile drains
                                       Constructed wetlands can also be used at the end of surface drains that collect and convey flows from several
                                       tile drains. Although predominantly receiving tile drainage, such surface drains are also likely to receive a
                                       component of surface drainage with higher particulate loads, particularly during intense rainstorms and
                                       extended wet periods. To deal with this additional sediment load from surface drainage, a sedimentation pond
                                       comprising an additional 10% of the wetland area should be included at the upstream end of the wetland (see
                                       page 21).

             To receiving water body   Scraping off the turf before constructing a wetland at the end of a surface drain, supplied by tile drains.

                                                                                                                   New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   8
Stage 2                             Design
This section provides guidance
for sizing and designing the
constructed wetland. It has been
broken down into nine sections:

A. Size
B. Layout
C. Embankments and sealing
D. Shape
E. Inlet structures
F. Sediment traps
G. Outlet and water-level control
   structures
H. Outlet protective structures
I. Phosphorus retaining additives

                                             New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   9
Stage 2Design
                                   A. Size: Constructed wetland size and treatment performance
  How big do I build it?
                                   The size of the constructed wetland depends on your treatment goals and available land area. Generally the
                                   bigger the wetland the better the treatment achieved. The following graph, developed from multi-year data
  Use this chart as a guide        from several different dairying regions, provides an estimate of average long-term nitrogen removal in relation
                                   to constructed wetland size.
  for determining how large
  your wetland needs to
  be to achieve your target                                                                   70%

  percentage nitrogen
  removal. Conversely, you         For tile drain                                             60%

                                                                                    removal
                                   flows from
  can use it to estimate the

                                                                                  removal
                                   grazed pasture                                                                                                                                           53 ±15%
  percentage removal you           in the North                                               50%                                                                                           removal

                                                                          nitrate-N
  can expect given your            Island of

                                                                        nitrate-N
                                   New Zealand.
  wetland size. The areas          (Performance                    annual
                                                                                              40%
                                                                                                                                       40 ±15%
  of wetland are given             in Southland                                                                                        removal
                                                                 annual

                                   and Otago
  as a percentage of the                                                                      30%
                                                     Percentage of

                                   expected to
                                                    Percentageof

  catchment area being             be ~ 5–8%                                                             22 ±10%
                                   lower across                                                           removal
  treated, therefore, you need                                                                20%
                                   range, due to
  to measure the catchment         lower average
  area that will drain into your   temperatures).                                             10%

  constructed wetland.
                                                                                              0%
                                                                                                    1%              2%                     3%                    4%                  5%
                                                                                                            Percentageof
                                                                                                         Percentage       drained
                                                                                                                    of drained    catchment
                                                                                                                               catchment in in wetland
                                                                                                                                             wetland

                                                                                                                         New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   10
Stage 2Design
                                                             A. Size: Calculating the wetland area required

                                                              Catchment area (ha) x % of catchment x 100 = Required wetland area (m2)
                                                                           1               x           2.5                x 100 =                                 250 m2

                                                             The chart below provides examples of wetland surface area for various catchment sizes:
                                                                                   Constructed
                                                              Catchment     % of
                                                                                   wetland area
                                                               area (ha) catchment
                                                                                       (m2)
                                                                   1            1              100
                                                                   1           2.5             250
                                                                   1            5              500
                                                                   5            1              500
                                                                   5           2.5          1250             A recently excavated, two stage, 250 m2 constructed wetland, with a mix of topsoil
                                                                                                             and subsoil returned into the base as the plant growth media.
                                                                   5            5           2500
                                                                  10            1           1000
                                                                  10           2.5          2500
                                                                  10            5           5000
                                                                  20            1           2000
Note: One hectare = 10 000 m2. The required ‘area’ of the         20           2.5          5000
constructed wetland applies to the base area of the actual
wetland; the embankments are additional to this.                  20            5           10000            The same wetland planted with raupo (Typha orientalis) after two seasons’ growth.

                                                                                                                      New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   11
Stage 2Design
                                                           B. Layout
                                                          The drawings below show a simple constructed wetland layout. An inlet structure distributes the inflow water
                                                          which then flows through the planted area before finally exiting through the outlet structure and discharging
                                                          to the receiving waterway.

                                                                        Longitudinal view                                                 Embankment
                             Inlet (rock rip-rap)         Maximum                                                                           planting
          ~0.8 m                                                                     Wetland planting
                                                          water level
                                                                                                                           Outlet                                           Discharge
                                                                                                                                                                                to
                                                         0.5 m                                                                                                               stream/
                                                      0.3 m                                                                                                                   drain
                  Tile
              drain outlet                            0.3 m

                       Growth media (30 cm) 50:50 topsoil/subsoil                       Compacted clay lining (15 cm)            Standard operating water level at low flow

  Inlet                                                                   Top view

                                                                                                                                                                            Stream/
                                                                                                                                             Outlet                          drain

  Excavating a wetland between a                                                                                   New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   12
  surface drain and receiving lake.
Stage 2Design
                                                                  C. Embankments and sealing
           Why do we need to                                      The embankment walls should be graded to provide a stable slope that will not erode or slump. This will
           seal the base of the                                   generally require a slope between 2:1 and 1:2 length-to-width ratio depending on soil structural properties.
           constructed wetland?                                   Appropriate vegetation of the embankments will help stabilise the banks.
                                                                  The base and lower 0.5 m of the embankments need to be sealed to retain the treated water and maintain
           To retain sufficient water                             sufficient moisture for survival of wetland vegetation. This can generally be achieved by compacting existing
                                                                  subsoils where the clay content is more than 10%. Compaction usually requires compression with a machine
           to keep the wetland                                    such as a digger, a sheep-foot roller or similar.
           vegetation alive and
                                                                  Where clay is not available, suitable clay will need to be imported from off-site or use of a geosynthetic clay or
           maintain its treatment                                 plastic liner (e.g., 250–500 micron low density polyethylene) considered. Plastic liners should be covered by at
           functions.                                             least 200 mm of soil to prevent root penetration and provide protection from sunlight UV damage. This will
                                                                  require initially excavating to a deeper depth, fitting the liner, and then covering it with soil to form the base of
                                                                  the wetland.

                                                                                                              Cross-sectional view
                                                                                                                         Width

                                                                                                  Standard low flow water level Maximum water level

                                                                             1m
                                                                                                                                  0.2 m
                                                                                                                                  0.3 m

                                                                                                                                 0.3 m

                                                                                    Compacted clay lining                        Growth media (30 cm) 50:50 topsoil/subsoil mix
Plastic liner used to make wetland water-tight in porous soils.

                                                                                                                           New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   13
Stage 2Design
  Rectangular shaped wetland              D. Shape: Constructed wetland shape
                Tile drains rerouted to
                pass through CW before    The shape of the wetland needs to provide the required
                entering into stream.     area (see page 10) and length-to-width ratio to achieve
                                          effective treatment. An elongated rectangular shape is
                                          generally the simplest way to achieve this, but there are                         Why is shape
                                          a range of other options available to achieve a similar                           important?
                                          outcome.

                                                                                   Recommended                              An elongated wetland
                                             Constructed wetland area
                                                                                 Length:Width ratio                         shape with the
                                                                                                                            inflow and outflow
                                               Up to 1000 m2 (0.1 ha)             3:1–5:1                               promote even flow
                                                                                                                            through the wetland.
                                          Very long narrow wetlands should be avoided because flow                          This reduces the
                                          velocities will become excessive during high-flow events.
                                Length

                                                                                                                            opportunity for short-
                                          Example of different length-to-width ratios for the same                          circuiting of flow or
                                          wetland area:                                                                     creation of flow dead-
                                                                                                                            zones, thus ensuring
                                            Constructed                               Length:Width
                                                             Length      Width                                              maximum treatment
                                            wetland area                                  ratio
                                                                                                                            efficiency.
                                                               100          10             10:1
                                              1000 m2           85          12             7:1
                Width                                           65          15             4:1
                                  Fence
                                                                                                  New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   14
Stage 2Design
    Free-form elongated shape       D. Shape: Alternative constructed wetland shapes
                       Tile drain   An alternative way to increase the length-to-width ratio of a wetland is to use internal bunds to create a hairpin
                                    flow path.

                                    Highly serpentine designs with multiple internal bunds are not recommended as tight corners tend to cause
                                    flow “dead-zones” which reduce the treatment efficiency of the constructed wetland. The shape of constructed
                                    wetlands does not need to be rectangular. More natural flowing shapes can fit better into the landscape.
                                    However, to maintain good flow along the wetland and avoid dead-zones, we recommend maintaining length-
                                    to-width ratios between 3:1 and 10:1, depending on the total area of the constructed wetland (see page 14).
                                    Also, the width should not be varied by more than 20% along the length of the wetland.

                                                                                                                                                 Hairpin shape

                                                                                                                                                               nd
                                                                                                                                                            B und
                                                                                                                                                           Bu
                                    Free-form shaped constructed wetland receiving multiple inflows, being                                                                 Tile drain
                                    built between a paddock and a lake.

                                                                                                             New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   15
Stage 2Design
                                                  D. Shape: Constructed wetland layout example
This constructed wetland is approximately
900 m2 and receives water from a 4.5 hectare
catchment. It is approximately 2% of the
                                                   1
catchment area, and has been divided into two
sections to utilise the natural lie of the land
and enable vehicle passage.

                                                            Lower wetland                                                             Upper pond wetland
                                                   2

                                                  Spring: 8 months after construction and planting.

                                                   3

                                                  Autumn: 28 months after construction and planting.

   Excavating a wetland between a                                                                      New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   16
   surface drain and receiving lake.
Stage 2Design
                                                                 E. Inlet structures
      Why are inlet structures                                   The performance of constructed wetlands is best when
      important?                                                 water flows evenly through the wetland utilising the full area
                                                                 available. Greater effort is required to disperse the inflow when
      Inlet structures help                                      wetland width and size are larger.
      distribute the in-coming                                   To contain the tile drain flow and provide support for the inlet
      water evenly across the                                    structure, the final 2 m of the perforated tile drain entering the
      constructed wetland.                                       inlet to the wetland should be rigid PVC pipe.
      This prevents erosion of                                   The table below summarises the recommended inlet structures
      soil and plants near the                                   to provide reasonable dispersal based on the constructed
      inlet. It also helps prevent                               wetland width.

      preferential flow channels                                                                                                                Schoenoplectus growing in a constructed wetland.
      forming. This increases the                                  Wetland width                     Inlet structure
      residence time of water in
      the wetland, enhancing                                                            Single central pipe with downward
                                                                     Up to 8 m          facing bend discharging into pile of
      removal of nitrogen and                                                           coarse rock rip-rap.
      other pollutants.
                                                                                        Inlet trench with discharge via
                                                                     8–30 m
                                                                                        submerged T-pipe.
Note: In situations where the tile drains flow into a surface
flow drain before reaching the wetland, it is recommended                               Split flow into two parallel wetlands of
that a transverse trench or pond be incorporated at the
inlet end of the wetland. This will act as a settling pond for     More than 30 m       lesser width using flow-splitting weir,
suspended particles transported via the surface drains (see                             use combination of above structures.
page 21).                                                                                                                                       Mature constructed wetland.
                                                                                                                           New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   17
Stage 2Design
                                                                 E. Inlet structures: Wetland width up to 8 m

                                                                                               Single pipe draining into pile of rip-rap

                                                                                                                                                                Tile drain

                                                                                                                                               Downward facing elbow on the
                                                                                                                                               end of the drain pipe to distribute
                                                                                                                                               water via the rip-rap pile

                                                                                      50–80 mm
                                                                                      rock rip-rap

                                                                    Water level

                                                                 A pile of 50–80 mm
                                                                 rip-rap under the subsurface
                                                                 drain is a simple method of slowing
                                                                 and distributing the inflowing water across
Example of a single drain pipe and simple T-pipe, see page 19.   constructed wetlands less than 8 m wide.
                                                                                                                  New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   18
Stage 2Design
         Submerged T-pipe                 E. Inlet structures: Wetland width 8–30 m
         Incoming tile drain pipe         A ‘T’ connector and two 0.5 m pipe extensions should be fitted onto the end of the subsurface drain pipe to
                                          deflect flow laterally across
                                          an inflow trench or pond.          Transverse deep-zone with submerged T-pipe inlet
                                          The transverse trench deep-zone
                                          length should be a minimum of
                                          5 m and span the width of the
                                          constructed wetland. It should                                                                                8–30
                                          be at least 1.2 m deeper than                                                                                        m
                                          the bottom of the main                                                                                            Top v
                                          constructed wetland                                                                                                     ie   w
           0.5 m       0.5 m              so that the water
                                          depth over                                                              0.5 m
                                          the trench is                         5m                                        0.5 m              1.2 m deep
                                          1.5 m deep.

                                          The
                                          deep
                                          zone
                                          also
                                          serves as a
                                          sedimentation                  Wat
                                          pond trapping                     er le
                                                                                    vel
                                          suspended particulates,                         30 c
      Warratahs or timber can be used     and may require periodic                               m
     to support the T-pipe. T-pipe must   cleaning out.
    be fixed to supports to ensure even
            flow from each side.
                                                                                                     New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   19
Stage 2Design
Once wetland widths approach 30 m we
recommend they are split into two parallel
                                                                 E. Inlet structures: Wetland width over 30 m
wetlands by building a central bund to
separate the two sides. The bund should                                 Flow-splitting weir
be a minimum of 0.8 m high. The inflowing
                                                                  Incoming tile                         Split flow into two parallel wetland
drainage water will need to be split between
                                                                  drains
the two wetlands. A well-designed and
                                                                                                   Flow-splitting weir
maintained flow-splitting weir will be required
                                                                                                                                                        T-pipe inlet structure
to provide even flow to each side.

                                                                  To constructed wetlands

                                                                                  Transverse
                                                                                  trench or pond

                                                                                                                         Dividing
Example of flow-splitting weir set in a sump with lid removed;                                                            bund
tile drainage water enters large chamber and is split between
the two outlets via V-notches.

                                                                                                     New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   20
Stage 2Design
                                     F. Sediment traps:
    Why settle out sediments?          Where wetland may receive sediment-rich inflows
    Fine soil particles suspended
                                     High sediment inputs can silt up shallow wetlands reducing their effectiveness and longevity. If there is potential
    in drainage water increase       for the constructed wetland to receive sediment-rich water from surface drains, flood flows, or other sources, an
    the turbidity of the receiving   inlet pond is necessary. It should be 1.5 m deep and cover an additional 10% of the surface area of the wetland.
    waterways. This leads to         The pond plays the same role as a transverse inlet trench, distributing the inflowing water evenly and allowing
                                     suspended sediments to be settled out and accumulated for later removal.
    smothering of the bottom,
    affecting aquatic organisms
    and recreational use.                                    Longitudinal section of transverse inlet trench/pond
    Sediments can also be an
    important source of nutrients
    such as phosphorus.                                                                      Minimum 5 m

                                                                                                                                                      0.3 m
                                                                                               Deep zone

                                                                                                                                                     1.2 m

                                                                                            Settled particles

Sediment trap being excavated.

                                                                                              New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   21
Stage 2Design
  Outlet pipe protective structures                       G. Outlet and water-level control structures
                   Timber chamber                         Stand-pipe: The height of the outlet structure determines the depth of water retained in the wetland. If
                                                          the width of the outlet zone is less than 8 m, a single adjustable standpipe, or adjustable weir (see page 23)
                                                          is generally sufficient. If the width of the wetland at the outlet end is wider than 8 m, a final 5 m-long outlet
                                                          trench is recommended, similar to those used for the inlet zone. This improves collection and transmission of
  Vertical                                     20 mm      flows from across the width of the wetland, increasing its hydraulic efficiency. An alternative option is to have
  timber                                       gaps       multiple outlet pipes or weirs at 6–8 m intervals across the width of the wetland outlet. However, these need
  slats                                                   to be carefully adjusted and maintained to ensure even outflow. Outlet pipes should be of similar or greater
                                                          diameter to that of the combined inflow pipes supplying them. This ensures adequate drainage at high inflows,
                                                          reducing the risk of overflowing the wetland (see note of caution, page 23).

                                                          An upturned PVC standpipe                           Adjustable stand-pipe
                                                          with outlet set 150 mm from
                                                          the bottom is necessary for            Normal operating depth
                                                          the first six months of plant                            150 mm
             Slotted plastic 200-litre drum               establishment. Once the plants        Plant establishment depth
                                                          have established, an additional                          150 mm
                                                          150 mm piece of pipe should
                                                          be fitted onto the existing one                                                    Riser pipe to be installed
                                                          to raise the water to an ideal                                                     after plants have established
                                        Attach drum                                                                                          to increase water depth.
                                        securely to       operating level of 300 mm.
                                        waratahs
                                                          Outlet protective structures: Outlet pipes or weirs should be protected from blockage by plant and other
                                            Outlet pipe   debris. The two methods we recommend for protective structures for a stand-pipe are a slatted timber
                                                          chamber or a slotted plastic drum. Cut slots or drill multiple holes of 40–60 mm in drum. The structure must be
                                                          secured to fence posts or waratahs so it does not float away. To further prevent pipe blockage, a 1 m-wide filter
                                                          zone of rip-rap can be incorporated around the structure. Removing debris from around protective structures is
Note: Use lid over structure to block sunlight and thus
minimise algal or weed growth on the outlet pipe.         easier than unblocking a pipe.
                                                                                                                  New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   22
Stage 2Design
                                                                 G. Outlet and water-level control structures
     Why are outlet structures
     important?                                                  Weir: An alternative to using outflow pipes is to construct adjustable height weirs using concrete and/or
                                                                 treated timber as shown below. Ensure that low water levels can be maintained during plant establishment,
     Outlet structures provide                                   and can be adjusted later. Care is needed to ensure stop logs are sealed so that water does not leak out around
                                                                 the edges, leaving the wetland plants dry between flow events. A triangular V-notch can be added in the centre
     control of the water level                                  of the weir to enable flow estimates. This requires addition of a staff gauge to measure the height of water
     within the wetland. Water                                   behind the weir. For further information see: www.engineeringtoolbox.com/weirs-flow-rate-d_592.html
     depth is especially important
     during plant establishment;                                                                         Adjustable weir
                                                                             Removable
     if it is too deep, the plants                                            timber or
     will float out or be drowned                                                metal
     before they establish.                                                     stop-log
                                                                               set in slot.

                                                                                              150 mm

                                                                                               150 mm
                                                                                                                                    Wooden outlet weir structure.

                                                                 Note of caution: Overflow is a threat if unusually heavy rainfall occurs that generates inflows that exceed the
                                                                 capacity of the outflow pipe, or the outlet is blocked. The risk of overflow is greater in situations where the
                                                                 constructed wetland receives a significant amount of surface flow as well as tile drainage flow. In these cases
                                                                 you should consider installing a high-flow bypass- an additional higher level outlet pipe or an area of rip-rap on
Outlet weir with one stop-log in place to control water level.   the constructed wetland embankment where the water can safely spill out.
                                                                                                                         New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   23
Stage 2Design
Other P-sorbing materials: A range of
P-sorbing materials potentially suitable for
                                                                    H. Phosphorus retaining additives
improving P retention in constructed wetlands
have been identified; these include: allophanic                     Reduction of phosphorus (P) losses from farms is also important in many New Zealand waterways. Wetlands
clays, lime, alum, smelter slag, and some volcanic                  are effective at settling out and retaining particulate phosphorus loads. However, in subsurface tile-drain
tephras (Ballantine, D.J.; Tanner, C.C. (2010)).                    flows, dissolved forms of P generally predominate. The capacity of wetlands to remove these dissolved forms
These materials are currently being assessed in                     of P is limited, with the main removal mechanisms being adsorption to soil minerals, uptake by plants, and
laboratory trials with a view to field-testing.                     accumulation within organic matter.

                                                                    Adsorption of P under waterlogged conditions is limited unless additional P-sorptive media are added to
                                                                                                        the wetland soils and/or used as porous filter materials. There is also
                                                                                                        potential for P release from constructed wetlands if topsoils with high
                                                                                                        P status are used as growth media. It is difficult to predict the P release
                                                                                                        characteristics of normal pasture soils once subjected to water-logged
                                                                                                        conditions in constructed wetlands. For farms with P-retentive allophanic
                                                                                                        and pumice soils, P loss via drainage systems is unlikely to be a significant
                                                                                                        issue and topsoils are likely to be relatively P-retentive under wetland
                                                                                                        conditions.

Constructed wetland planted with sedges and rushes.                                                       We recommend the following measures to increase P retention in
                                                                                                          constructed wetlands treating tile drainage, particularly in P-sensitive
*Note: The use of subsoils with lower organic matter content                                              catchments:
is likely to reduce N removal performance during the first few
years of operation. This will be overcome as wetland plant litter                                         1. Where possible use a 50:50 mix of subsoil from the B or C horizon
accumulates in the wetland providing a sustainable ongoing                                                   mixed with topsoil as the growth media in the base of the wetland.
supply of organic matter for denitrifying bacteria. Further                                                  This mix will reduce the overall P status of the wetland soil and
studies are currently underway and recommendations will
                                                                                                             introduce more P-retentive material to the soil. If allophanic soils or
be reviewed when further information becomes available.
                                                                                                             subsoils can be brought onto the site from nearby areas, these should
**See following website for general distribution of                                                          be used in preference and/or combined with the local soil*.
allophanic soils in New Zealand:
http://soils.landcareresearch.co.nz/contents/                                                             2. Additionally, incorporate agricultural lime at a rate of 0.5 kg per m2
and follow links:                                                                                            into the top 50 mm of soil**.
Soil Names à Soil classification (NZSC) à Soil Order                Cyperus ustulatus

                                                                                                                           New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   24
Stage 3                          Construction
This section provides guidance
for building the constructed
wetland.

                                                New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   25
Stage 3Construction
                                                             A. Timeline/planning
                                                             Things to consider during the pre-construction stage:

                                                             1. Do you need permission?         Check with your regional council that this is a permitted activity and for
                                                                                                any specific restrictions or rules that need to be followed.

                                                             2. Identify:                       An experienced earthmoving contractor and make sure they know what is
                                                                                                required to construct a wetland as outlined in this guide.

                                                             3. Make sure conditions are         The soil should be dry enough for the digger to properly work the soil
                                                               sufficiently dry for earthmoving: materials and avoid getting stuck.

Excavating a constructed wetland.                            4. Pre-order plants:               It is wise to identify a supplier of wetland plants and speak to them about
                                                                                                the types, quantities and timing of what you need several months in
                                                                                                advance. This way they should be available when you need them (see
                                                                                                Appendix 1).

                                                             5. Purchase:                       Purchase supplies for building the inlet and outlet structures.

                                                             6. Planting:                       The ideal time to plant is spring/early summer when there is still enough
                                                                                                water coming out of the drains to supply the constructed wetland. It also
                                                                                                gives the plants a head start to establish themselves before winter,
                                                                                                improving their chances of survival.

                                                             7. Supplementary water supply:     Often the best time of year for construction is in the summer when it is too
                                                                                                dry for planting. If this is the case, you must have a supplementary water
                                                                                                source available to keep the wetland moist to allow the plants to establish
                                                                                                (see Stage 4). Otherwise planting should be delayed until conditions are
                                                                                                suitable.
Mix of topsoil and subsoil placed into bottom of excavated
constructed wetland.

                                                                                                                     New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   26
Stage 3Construction
                                                                   B. Construction day and beyond
                                                                   1. Before the digger arrives, peg out exactly where
                                                                      you want the constructed wetland to be dug.

                                                                   2. The turf layer should initially be scraped off and
                                                                      disposed of. The topsoil should then be removed
                                                                      and set aside in a separate pile for later return as
                                                                      a growth media.

                                                                   3. Excavate the wetland to the required depth.
Rectangular constructed wetland planted with raupo.
                                                                      If clay is available, compact the base of the
                                                                      wetland, otherwise seal with the appropriate
                                                                      material (see page 13).

                                                                   4. Once excavated, lined and shaped, cover the
                                                                      base with the set-aside 50:50 mix of topsoil
                                                                      and subsoil to a depth of 30 cm. Ensure that
                                                                      this growth media is only lightly packed down,
                                                                      suitable for planting into.

                                                                   5. Install the inlet and outlet structures.

                                                                   6. Plant the plants and flood with the appropriate
                                                                      amount of water (see Stage 4).

Digger creating a pile of topsoil for later use as growth media.                                                                Starting excavation of a constructed wetland.

                                                                                                                             New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   27
Stage 4                          Establish plants
This section provides guidance
for planting the constructed
wetland including why, what,
and how to plant.

                                                New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   28
Stage 4Establish plants
                                  A. Role of plants
   When should I organise
   the plants?                    Constructed wetlands work best with a well-maintained, dense cover of emergent wetland plants. Plants are
                                  crucial to the functioning of constructed wetlands because they:

   You should start organising    t   provide the physical structure that supports the growth of
   the plants early on in your         microbial biofilms, which are important in their functioning
   planning. It is likely you     t   produce litter as a source of organic matter for
   will need to order at least         denitrification and other microbial processes
   a portion of your plants       t   shade the water surface to reduce algal growth
   from a specialist wetland      t   dissipate wind and wave action to reduce erosion
   nursery which may have
                                  t   enhance wildlife and aesthetic values.
   to grow them for you.
                                                                                                         Carex species plants shading constructed wetland surface.
   You may also be able to
   harvest wetland plants for
   transplanting from existing
   wetland areas on your
   farm. Ensure you do not
   damage valuable natural
   wetlands, or transfer weeds
   along with the intended
   species, particularly in the
   soil around the plant roots.                                                                          Raupo litter providing organic matter to support
                                  Schoenoplectus species dissipate wave action.                          denitrification.

                                                                                          New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   29
Stage 4Establish plants
Key species* for:
                                                             B. Planting zones
Flooded zone
t Raupo (Typha orientalis)
                                                             There are three main regions within the constructed wetland:
t Kapungawha (Scheonoplectus
    tabermontani)                                            1. the flooded zone
t Mokuautoto (Baumea articulata)
t Kuta (Eleocharis sphacelata)                              2. the shallow margins

Shallow margins and embankments                              3. the embankments.
t Harakeke, NZ Flax (Phormium tenax)
                                                             Water level is the differentiating factor with the flooded zone being the wettest and the embankment being
t Toetoe (native Cortaderia species)
                                                             the driest. The plants in the flooded zone are primarily responsible for water treatment while the plants on the
t Purei (Carex secta)
                                                             margins and embankment stabilise the edges, help exclude weeds, and promote biodiversity.
*Maori names for some native plants differ with region. We
have attempted to use the most widely recognized name
where possible.

                                                     Constructed wetland cross-section illustrating planting zones

     Embankment

                       Shallow-water margins                                                                                                        Flooded zone

                                                                                                                     New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   30
Stage 4Establish plants
                                                     C. Plant selection: Key species for the flooded zone
                                                     The tall, emergent plant species listed below grow well within the flooded zone of constructed wetlands. Native
                                                     species adapted to your locality are recommended. Raupo is the plant of choice for treating farm drainage
                                                     high in nitrate-nitrogen. It is easy to establish and fast-growing, producing abundant plant litter to stimulate
                                                     nitrate-removing bacteria. It is relatively resistant to weed invasion and tends to dominate other species under
                                                     nutrient-rich conditions. Alternatively, plant a combination of the other species as listed below.

                                                           Plant              Common                 Natural                                                                                Depth
                                                                                                                                             Description
                                                          species              name                   range                                                                                 range

                                                                            raupo, bulrush                              1.5–3 m tall. Dull green, erect leaves arising in clumps
                                                                             equivalent to:                          from green spongy rhizomes. Thick, cylindrical brown seed
                                                                                                    Throughout
                                                       Typha orientalis    cumbungi (Aus),                               heads (cat-tails) borne on tall shoots. Leaves die back           0–0.4 m
                                                                                                    New Zealand
                                                                           reed mace (UK),                            strongly in winter. Generally dominant emergent wetland
                                                                          cattail (N. America)                              plant in fertile lowland New Zealand swamps.

                                                                                                                        1.8 m tall. Green year-round. Dark green, ‘leafless’, stiff,
                                                                             mokuautoto,           From Northland
                                                      Baumea articulata                                                cylindrical shoots with obvious joints when mature. Red-            0–0.4 m
                                                                           jointed twig-rush          to Levin
                                                                                                                    brown pendulous seed heads borne on separate fertile shoots.

                                                                                                   Throughout NZ
                                                                                                     (common in          0.8–1.3 m tall. Stout, bright green ‘leafless’, hollow
                                                         Eleocharis       kuta, tall spike-rush,
                                                                                                    North Island,      shoots with transverse internal septa, arising from thick          0.2–0.6 m
                                                         sphacelata           spike-sedge
                                                                                                    uncommon in            rhizome. Seed heads forming at tip of shoots.
                                                                                                     Canterbury)

                                                                                                                        0.6–1.8 m tall. Shoots die back over winter, except in
                                                                            kapungawha,             Northland to     northern coastal areas. Erect green to blue-green, ‘leafless’,
                                                       Schoenoplectus
                                                                          soft-stem bulrush,        Westland and       cylindrical shoots with white central pith, arising from            0–0.4 m
                                                      tabernaemontani
                                                                            lake clubrush            Canterbury      horizontal rhizome. Brown seed heads form tuft just below
                                                                                                                       the shoot tip. Shoots die back in winter at inland sites.
Baumea articulata (mokuautoto, jointed twig-rush).

                                                                                                                         New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   31
Stage 4Establish plants
Key flooded zone species: Typha orientalis (raupo, bulrush)

                                                              New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   32
Stage 4Establish plants
Key flooded zone species: Baumea articulata (mokuautoto, jointed twig-rush)

                                                          New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   33
Stage 4Establish plants
Key flooded zone species: Eleocharis sphacelata (kuta, tall spike-rush)

                                                              New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   34
Stage 4Establish plants
Key flooded zone species: Schoenoplectus tabernaemontani (kapungawha, soft-stem bulrush)

                                                          New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   35
Stage 4Establish plants
C. Plant selection: Key species for shallow-water margins and embankments
These species are suitable for planting in shallow water and on embankment slopes to reduce bank erosion and weed ingression, and enhance plant and habitat
diversity. For identification of these species, see ‘Wetland Plants in New Zealand’ (Johnson and Brooke, 1989).

      Plant            Common             Natural                                   Planting
                                                            Description                                        Comments                                            Photo
     species            name               range                                    position

                       purua grass,                           1–1.8 m tall.
  Bolboschoenus         kukuraho,       Northland to   Leafy sedges with stems,                 Common in coastal areas. Fast-growing
                                                                                 Shallow water
  fluviatillis and    ririwaka, river   Westland and     (triangular in cross-                   in spring and early summer, dies back
                                                                                to 0.3 m depth.
    B. medianus           bulrush,       Canterbury    section), emerging from                  over winter. Provides seasonal diversity.
                     marsh clubrush                     woody, bulbous tubers.

                                                             1–1.5 m tall.        Moist lower     Establish initially in moist conditions
                                                           Drooping harsh        embankments      or shallow water, can grow in deeper
                                        Throughout
    Carex secta      purei, makura                     tussocks forming trunk-    and shallow     water if gradually acclimatised. Classic
                                        New Zealand
                                                       like base when mature.       water to       New Zealand plant of wetland and
                                                          Green year-round.       0.2 m depth.                stream margins.

 Other Carex spp.;                                                                Moist lower
                                                                                                  Taller-growing species mentioned are
      especially                                           0.5–1.5 m tall.       embankments
                                        Throughout                                                 likely to be the most robust and able
   C. germinata,      rautahi, carex                     Harsh leafy sedges.      and shallow
                                        New Zealand                                                to compete with weeds. Valuable for
 C. lessoniana and                                       Green year-round.          water to
                                                                                                                   wildlife.
      C. virgata                                                                  0.2 m depth.

                                                                                                                  New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   36
Stage 4Establish plants
C. Plant selection: Key species for shallow-water margins and embankments

       Plant              Common            Natural                                     Planting
                                                               Description                                         Comments                                            Photo
      species              name              range                                      position

                            toetoe
                        (New Zealand        Different           1.5–3 m tall.
                                                                                      Upper & lower    Useful, hardy plant suitable for bank
Cortadera richardii,    native species       species      Coarse green tussocks,
                                                                                      embankments       stabilisation and screening. Ensure
    C. fulvida,         only, not to be     common           with tall feathery
                                                                                      to water edge    invasive introduced pampas species
     C. toetoe          confused with      in different   flower heads borne on
                                                                                      and surrounds.                are avoided.
                         introduced          regions         cylindrical stems.
                       pampas grasses)

                                                                                          Upper
                                                               Tall-growing
                                                                                      embankments
                          ti kouka,       Throughout        soft-stemmed tree                          Classic New Zealand tree common in
 Cordyline australis                                                                       and
                        cabbage tree      New Zealand     bearing tufts of fibrous                                  wet soils.
                                                                                       surrounding
                                                                   leaves.
                                                                                          areas.

                                           Northland to         0.5–1 m tall.      Moist lower
                       toetoe upoko-        Canterbury    Harsh pale-green leaves embankments           Tolerates dry periods. Suitable for
 Cyperus ustulatus      tangata, giant    and Fjordland; in clumps, with emergent and shallow          wetland margins and embankments,
                       umbrella sedge     mainly coastal     seed-bearing leafy      water to                  and shallow water.
                                           and lowland            umbells.         0.2 m depth.

                                                                1–3 m tall.
                                                                                      Upper & lower     Does not generally establish well in
                                                          Robust clumps of tough
                          harekeke,       Throughout                                  embankments      continuously flooded conditions. An
 Phormium tenax                                            robust leaves. Tall dark
                       New Zealand flax   New Zealand                                 to water edge    important traditional plant for Maori
                                                           brown to black flower
                                                                                      and surrounds.    and important habitat for wildlife.
                                                                   heads.

                                                                                                                      New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   37
Stage 4Establish plants
                                                   D. Planting and establishment
                                                   Care is needed to ensure rapid establishment of wetland plants and maximise the survival rate before weeds
                                                   invade. Below are some tips for successful planting:

                                                   1. Timing of planting
                                                      Most wetland plants do not grow much during winter and in many the above-ground portions die back
                                                      over this period (e.g., raupo). Planting should, therefore, ideally take place in spring or early summer
                                                      (September–December) to promote rapid establishment and enable growth of a tall dense cover that can
                                                      outcompete weeds. However, planting at this time is often difficult in practice, because ground conditions
                                                      remain too wet for construction early in the season.
Planting Schoenoplectus into flooded zone.
                                                      Later planting in summer (January–February) is possible if larger plant grades are used and a
                                                      supplementary water source is available to keep the wetland moist. Planting later in the season, or without
                                                      such supplementary care is not recommended. Instead, it is better to wait until the following spring to
                                                      undertake planting.

                                                   2. Pre-planting weed and pest control
                                                      If the area is weedy initially or has been left for any time, it will need manual weeding, or spraying
                                                      with a translocated herbicide such as glyphosate. If large masses of herbicide-killed plant material are
                                                      subsequently flooded they will decompose creating de-oxygenated conditions detrimental to plant
                                                      establishment. It is therefore advisable to control weeds at low levels (to avoid build-up of biomass) and/
                                                      or physically remove any accumulated dead plants. Useful resources to help identify and control weeds are
                                                      noted in Appendix 2 and Appendix 3.

                                                      Pukeko and Canada Geese, in particular, can be very destructive to newly planted wetlands and may
                                                      need controlling during early plant establishment. There are a range of bird deterrents that may be useful
                                                      including gas bangers and flashing lasers. If you intend to dissuade birds by shooting, you will need a
                                                      hunting permit from The Fish and Game Council and possibly also a special permit to extend control
Tray of Schoenoplectus ready to be transplanted.      beyond the open game-bird hunting season, www.fishandgame.org.nz/Site/Regulations/default.aspx.
                                                                                                         New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage   38
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